If you asked the average Pennsylvanian why they were celebrating the 4th of July this year, they would probably tell you that it was to mark Independence Day. But if you asked the same question to Elliot Lipeles, associate professor of physics in the School of Arts & Sciences, he might have another reason: July 4th, 2022, was the 10th anniversary of the discovery of the Higgs boson.
Lipeles is a particle physicist whose work has taken him to the Large Hadron Collider (LHC), the world’s largest and most powerful particle collider that spans the border between France and Switzerland. He joined an LHC project known as the ATLAS experiment in 2009, right as the European Organization for Nuclear Research (CERN) began collecting data to confirm the existence of the Higgs boson, physics’ most elusive particle yet.
Before that, Lipeles worked at the competing LHC experiment, CMS, just a few miles over the French border. Because each project partners with different institutions, he switched to the ATLAS experiment after joining Penn’s physics department. The two experiments utilized different equipment and data analysis methods, but both were on the hunt for the Higgs boson together. “It’s a friendly rivalry,” says Lipeles.
Discovering the Higgs
The Higgs boson, also called the Higgs particle, is key to the Standard Model of physics, which describes a framework for how fundamental forces and subatomic particles form the universe. While the role of the Higgs is complex, “the first thing people usually say is that it gives particles mass,” Lipeles says. Theoretical physicists proposed the existence of the Higgs boson in 1964, but it took decades for experimental research to catch up.
Lipeles says the Higgs boson “was actually particularly difficult to find.” That’s because it is heavier than other subatomic particles, and heavy subatomic particles don’t tend to stick around for long. Instead, they decay into smaller, lighter particles, such as photons.
So physicists needed to detect traces of the Higgs without ever actually seeing it. That’s where the LHC—and physicists like Lipeles—came in.
The LHC works by colliding subatomic particles at nearly the speed of light in hopes of creating a Higgs boson. While the Higgs won’t linger long enough for the physicists to ever detect it directly, they can predict which particles the Higgs will decay into, then look for those specific particles.
Fellow Penn physicists Brig Williams, Evelyn Thomson, and Joseph Kroll led the development of certain electronic components in ATLAS’ detector, a device similar to a camera that takes millions of subatomic snapshots every second. But if ATLAS kept every single picture from their detector, they would be left storing a volume of data equal to eight times the Library of Congress’s collections every second.
So, Lipeles helped develop a “trigger” system for the detectors that stored only the most important images, about one in every 100,000. The device is called a trigger because it automatically detects which images to discard and which to keep, then instantly triggers data storage of those pictures.
The trigger system at ATLAS is what helped LHC physicists uncover the subatomic particles appearing at just the right amount of mass and energy to indicate the existence of the Higgs boson. After years of data collection, the ATLAS experiment finally passed the high bar set by physics for discovery of “five sigma,” meaning that there was only a 1 in 3.5 million chance the observed data was up to chance and not indicative of the Higgs’ existence.
To announce the discovery, CERN called a meeting in an auditorium at the LHC on July 4, 2012. “There was a line out the door to get in,” says Lipeles. When ATLAS and CMS experiment leaders announced that the data they’d gathered passed the five sigma threshold, the crowd cheered. Photographs of the milestone show a Penn flag hanging on the auditorium wall. Theorists Peter Higgs and Francois Englert won the 2013 Nobel Prize for their founding role in the experiments.
Physics since the Higgs
Since the existence of the Higgs was confirmed, the field of particle physics has gone through “a lot of big changes,” says Lipeles. While the Higgs helps fit some of the mathematical formulas established in the Standard Model of physics, questions remain about how the numbers add up. Those questions, says Lipeles, make some physicists think there are other particles waiting to be discovered.
Exactly what those new particles are and what they do is one of the next problems Lipeles is tackling. One solution might be “supersymmetry,” which “predicts a whole new set of particles for every particle we already have,” he says. Each proposed particle would correspond to an existing particle but with slightly different subatomic properties. These new, supersymmetric particles would help explain some outstanding questions about the Higgs boson.
Lipeles is also examining how the Higgs might interact with dark matter. Dark matter is a hypothetical, invisible form of matter initially proposed by astrophysicists to explain large-scale gravitational forces in galaxies. Now, particle physicists at the LHC are considering if dark matter might explain remaining mysteries of the Higgs, too.
In order to conduct these new experiments, the ATLAS project is gearing up to record more data and more collisions than ever. All those new collisions will emit a lot of radiation that would destroy the original detectors, Lipeles says. “So we have to make a new detector that can separate out more small things, and it has to survive more radiation.”
Lipeles describes this new generation of detectors he’s helping to build like a high-tech onion, with layers that expand out from the core of the collider. Each layer uses a different type of technology to pinpoint signatures of the subatomic particles generated by the collisions.
“The second layer out is what Penn is working on,” he says. The device will be a strip about 5 centimeters long that takes even more high-resolution snapshots than the last “camera.” And the device needs to be sturdy, too; once the collisions start, it will become so heavily irradiated that researchers won’t be able to touch it again.
Lipeles spends more time in Pennsylvania now teaching, while a group of new researchers represent the next generation of Penn scientists at the LHC. That group includes graduate students Gwen Gardner and Lauren Osojnak, as well as postdocs Jeff Shahinian and Nadezhda Proklova.
When Lipeles reflects back on his own time as a physics student, he describes the excitement of learning each new theory and branch of physics. “And then you get to a point where it’s like, ‘Now what?’ We don’t know. Nobody knows why the Higgs boson is like this. Nobody knows what dark matter is.”
Lipeles isn’t letting the unknown stop him, though. “You’ve got to go out there and look right into it. That’s the continuation of the story.”